FIG. 11 schematically shows an optical communications link. An optical communications link 3 includes an optical fiber 2 for transmitting light that has been modulated according to a data signal to be transmitted, and optical communications modules 1 that are connected to the both ends of the optical fiber 2 by optical coupling.
The optical communications link 3 can be classified into a number of categories based on the mode of communication. Namely, the optical communications link 3 can be roughly classified into the following three categories: (1) an optical communications link using optical fiber 2 with a single core or multiple cores; (2) an optical communications link in which signals are transmitted bidirectionally or unidirectionally; and (3) an optical communications link in which signals are transmitted simultaneously in full-duplex or semi-duplex. These different modes are often combined, for example, as in a single-conductor full-duplex communication mode, to carry out optical communications.
The optical communications module 1 requires a light emitting element and a light receiving element for bidirectional communications of signals. For unidirectional communications, the optical communications module 1 requires either one of the light emitting element and the light receiving element.
Note that, as the terms are used herein, the light that emerges from the light emitting element of the optical communications module 1 and enters an end portion of the optical fiber 2 will be referred to as “outgoing light”, and the light that emerges from an end portion of the optical fiber 2 and enters the light receiving element of the optical communications module 1 will be referred to as “incoming light”.
Incidentally, Japanese Publication for Unexamined Patent Application No. 65208/1986 (Tokukaisho 61-65208; published on Apr. 3, 1986) and No. 234060/1996 (Tokuhaihei 8-234060; published on Sep. 13, 1996) disclose a method by which coupling efficiency (transmission efficiency) of transmitted light and optical fiber is improved by way of increasing end faces of the optical fiber.
That is, this method increases end faces of the optical fiber to enable transmitted light to be easily and efficiently coupled even when the core diameter of the optical fiber is small as in a quartz optical fiber. In this way, the method improves tolerance characteristics for misalignment.
The foregoing publications are merely concerned with improvement with regard to a coupling method of optical fiber and light emitting element and they do not describe anything about improving reception efficiency, or a structure that can accommodate to a single-conductor full-duplex mode. In particular, the publications do not provide a method of efficiently separating incoming light and outgoing light from each other and preventing interference of incoming light and outgoing light.
One problem of a conventional full-duplex communication mode using multiple optical fibers is that it is difficult to miniaturize the optical communications module and the cost of the optical fiber is increased as the transmission distance becomes longer. In order to solve this problem, there has been proposed an optical communications module for a single-conductor full-duplex mode, in which a single-conductor optical fiber is used to send and receive signals simultaneously using the light of the same wavelength.
Particularly notable in this regard are plastic optical fibers (“POF” hereinafter). POF has advanced over the last years to attain smaller loss and wider band and are now being applied to home communications and inter-device communications of electrical devices. POF has a large core diameter of about 1 mm, which allows for easy coupling with the optical communications module. This enables the POF to be easily plugged in and out of the optical communications module, thereby providing a user-friendly optical communication link.
In the optical communications module that carries out full-duplex communications using a single-conductor optical fiber and the light source of the same wavelength, it is important to prevent interference of outgoing light and incoming light. Some of the causes of interference between incoming light and outgoing light are:
(1) Reflection at an end face of the optical fiber when outgoing light enters the optical fiber (hereinafter “near-end reflection”);
(2) Internally scatted light within the optical communications module (hereinafter “internally scattered light”);
(3) Reflection in the optical communications module of the other end in communications (hereinafter “module reflection at the other end”); and
(4) Reflection of outgoing light at an end face of the optical fiber when it emerges from the optical fiber (hereinafter “far-end reflection”).
In addition, (5) electrically induced interference also causes problems.
In the optical communications link using an optical fiber as a transmission medium, it is also important to efficiently couple incoming light from the optical fiber with a light receiving element, so as to obtain a high S/N (Signal to Noise) ratio.
An example of the conventional single-conductor full-duplex optical communications module can be found, for example, in Japanese Publication for Unexamined Patent Application No. 153720/1998 (Tokukaihei 10-153720; published on Jun. 9, 1998), which describes a method of separating outgoing light and incoming light from each other using a polarized light separator.
Namely, this method works under the principle that the direction of polarization of outgoing light is random in the course of travel through the optical fiber, while the outgoing light reflected at the end face of the optical fiber (near-end reflection) has the same polarization direction. Thus, by interposing the polarized light separator that reflects only polarized light between the optical fiber and the light receiving element, interference by the near-end reflection can be prevented.
However, owning to the fact that about half of the outgoing light is reflected by the polarized light separator, a reception loss of about 3 dB is generated and the outgoing light cannot be used efficiently. Further, because the outgoing light is polarized, it is difficult to use an inexpensive light emitting diode (LED) as the light emitting element.
A solution to this problem is described in Japanese Publication for Unexamined Patent Application No. 27217/1999 (Tokukaihei 11-27217; published on Jan. 29, 1999) and No. 352364/1999 (Tokuhaihei 11-352364; published on Dec. 24, 1999). These publications disclose a method in which outgoing light is incident off-center on the optical fiber and the incoming light that emerges from the other area of the optical fiber is received. This method is described below referring to FIG. 12 and FIG. 13.
Outgoing light 121 from a light emitting element 104 is converged by a sending lens 106 and reflected by a guiding mirror 107 into an end face 108 of an optical fiber 102, off-center from the optical axis. Incoming light 122 that emerges from the end face 108 of the optical fiber 102 is coupled with a light receiving element 105.
FIG. 13 shows how the coupling position of the outgoing light 121 on the end face of the optical fiber 102 is related to a receiving area. In the method in which the outgoing light 121 is incident off-center on the optical fiber 102, the sending area on which the outgoing light 121 is incident is spatially separated from the receiving area on the end face of the single-conductor optical fiber 102. In this way, single-conductor full-duplex communications are realized.
The method, by the larger receiving area than the sending area, is able to separate the outgoing light and the incoming light from each other with a small loss, smaller than 3 dB that is generated in the method using the polarized light separator. As a result, reception efficiency is improved.
Despite improved reception efficiency, the outgoing light 121 still needs to be incident within the core diameter of the end face of the optical fiber 102 for successful coupling with the optical fiber 102. It is therefore necessary, considering a tolerance for axial misalignment of the optical fiber 102 or an assemble tolerance of the optical communications module, that the outgoing light 121 be incident on a position that takes into consideration a tolerance margin from the periphery of the optical fiber 102.
For this reason, it was difficult to provide a sufficiently large receiving area, which made it difficult to provide sufficiently high reception efficiency. Further, a faster communication speed requires a wider communication band. The wider band in a receiver circuit increases electrical noise. Given this and to maintain a sufficient S/N ratio, higher reception efficiency is required.